1. Field of Invention
The invention relates to a motor speed control circuit and, in particular to a motor speed control circuit having a synchronous pulse width modulation (PWM) signal, which is able to control the motor to operate smoothly such that audible noise is effectively reduced.
2. Related Art
To reduce the power consumed in coils of a DC motor or a brushless fan, pulse width modulation (PWM) control techniques are now used in place of linear driving techniques. By applying PWM control techniques to the DC motor, the coils of the motor are efficiently driven, and the power consumption is able to be kept at a minimum.
With reference to FIG. 7, the Hall signal responds to the rotation speed of the motor, i.e., if the rotation speed increases, the frequency of the Hall signal becomes higher. Conventionally, the frequency of a PWM control signal is a constant, and the frequency of the PWM control signal is different from the Hall signal, therefore the PWM control signal is not synchronized with the Hall signal. Thus, audible noise is generated when the motor is operated, especially at low speed.
To overcome the shortcomings, the present invention provides a motor speed control circuit having a synchronous PWM signal to effectively mitigate and obviate the aforementioned problems.
The objective of the invention is to provide a motor speed control circuit having a synchronous PWM signal to effectively reduce audible noise when the motor is operating.
To achieve the foregoing objective, the PWM control circuit mainly comprises a synchronous phase detecting unit and a PWM control unit to generate a synchronous control signal applied to the motor.
Other objects, advantages and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.